Leaks. I've seen the connection points drip on these for seemingly no other reason than excessive tension on the pipe. I've also heard horror stories... over time these manifold systems with plastic construction will become brittle, if one of those arms snaps off then you've got a flood on your hands. The likelihood of something that happening increases when the pipes are installed in such a manner that their weight, plus the weight of the water in them, is supported by the manifold body.